1. Field of the Invention
The present invention relates to an optical system, which is suitable for use in an optical apparatus, such as a silver-halide film camera, a digital still camera, a video camera, a digital video camera, a telescope, binoculars, a projector, and a copying machine.
2. Description of the Related Art
Conventionally, in an optical element (lens) made of a transmissive medium, such as glass or plastic, an increase of surface reflection often causes flare and ghost and reduces a transmittance thereof. Therefore, the optical element (lens) is coated with an antireflection coating made of a thin film made of dielectric film on the surface thereof.
It is required that the antireflection coating has an excellent antireflection effect even when a range of incident angles of light fluxes incident on the optical element is wide.
In order to obtain a high antireflection effect in a wide incident angle range, a difference in refractive index between air and a layer, and between layers must be low. Accordingly, it is effective to use a functional film having a refractive index lower than that of the dielectric film.
As the functional film having a low refractive index, a porous film in which fine cavities are dispersed in a layer (in transparent material) and a film including fine particles (porous optical material) are discussed in Japanese Patent Application Laid-Open No. 6-003501 and Japanese Patent Application Laid-Open No. 2004-302112.
In addition, a functional film having a fine concavo-convex structure which is less than a wavelength of visible light and having an effect of antireflection is discussed in Applied Optics, Vol. 25, No. 24, pp 4562 to 4567 (1986).
In order to incorporate and hold a lens in a lens barrel, the lens is incorporated in the lens barrel and is fixed via a holding member (presser ring). Further, a holding member is used to hold the lens, while a relative clearance between adjacent lenses is maintained via a spacer.
In this case, in order to shorten the length of the entire optical system and to improve the assembly accuracy, a method (hereinafter, referred to as marginal contact) can be used in which a lens is directly made contact with an adjacent lens at the periphery thereof and the state is held in a lens barrel.
Normally, the accuracy of processing a barrel and a spacer made of a metal or a molding resin is from several microns to several ten microns. Compared with this, the accuracy of processing a curvature of a lens surface needs accuracy of sub micron order. Accordingly, a marginal contact method is extremely effective when the accuracy of clearance between two opposing lenses and the accuracy of decentering are highly required, and can be widely applied to an optical system where high accuracy is required.
Further, some lenses having high sensitivity with respect to the decentering of a lens is assembled by fixing it after performing the decentering adjustment in a lens barrel.
FIGS. 8A and 8B are diagrams illustrating a method for fixing two lenses 1 and 2 in marginal contact with each other in a lens barrel 3 after performing a decentering adjustment to an optical axis 9, and then fixing it.
FIG. 8A is a diagram illustrating a state of performing a rotation adjustment of the two lenses 1 and 2 in marginal contact with each other as indicated by an arrow in a plane orthogonal to the optical axis 9 to perform aligning.
FIG. 8B is a diagram illustrating a state of performing a shift adjustment of the two lenses 1 and 2 in marginal contact with each other as indicated by an arrow in a plane orthogonal to the optical axis 9 to perform aligning.
The functional film with a low refractive index for obtaining an excellent antireflection effect in the range of a wide incident angle discussed in Japanese Patent Application Laid-Open No. 6-003501, Japanese Patent Application Laid-Open No. 2004-302112, and Applied Optics, Vol. 25, No. 24, pp 4562 to 4567 (1986) has extremely low film hardness compared with a film having a low refractive index made of a dielectric film.
Thus, when two lenses have marginal contact with each other to shorten the whole length of an optical system and to obtain high assembly accuracy, if a lens surface with a functional film thereon contacts a surface of another lens, the functional film may be damaged at a contact portion and the antireflection effect may be reduced.
Further, a part of a damaged functional film may be scattered on surfaces of the lenses and it may cause a deterioration of optical performance.
Accordingly, it has come to a significant purpose to obtain an optical system that a functional film is not damaged when a lens with a functional film thereon has marginal contact with another lens and is incorporated in a lens barrel.